Evaporable foam patterns made of polystyrene, or the like, are frequently employed in casting operations, particularly when casting articles of complex configurations, such as internal combustion cylinder blocks or other engine components. In the typical evaporable foam casting process, the pattern, which conforms in shape to the metal part to be cast, is positioned in an outer mold and a flowable material, such as sand, is introduced into the mold to surround the pattern and fill the cavities of the pattern. When molten metal is introduced into the mold, the metal will vaporized the foam pattern, with the vapor passing into the interstices of the sand while the metal fills the void produced by evaporation of the foam pattern, thereby resulting in a cast metal part having a configuration identical to the evaporable foam pattern.
The evaporable foam pattern for a two-cycle engine generally includes an cylinder block pattern section, an exhaust passage cover section, and a water passage cover section, which are joined together by a glue or adhesive along mating interfaces. The cylinder block pattern section has an outer peripheral rim and an internal wall spaced inwardly from the rim to define a water passage, while the area within the internal wall constitutes an exhaust manifold passage which communicates through exhaust ports with the cylinders of the engine.
The exhaust passage cover section also includes a rim that abuts the rim on the engine block section and the inner surface of the exhaust passage cover section is formed with an internal wall that is spaced inwardly from the rim and abuts the internal wall of the cylinder block section. The central portion of the exhaust passage cover section encloses the exhaust manifold passage in the engine block section.
The water passage cover section has a peripheral rim that is in abutting relation with the rim of the exhaust passage cover section, and the central portion of the water passage cover section, located within the rim defines a water chamber which communicates with the peripheral water passage of the exhaust passage cover section.
The pattern sections are joined together by applying a layer of glue or adhesive to the mating rim surfaces, as well as to the mating internal walls between the cylinder block section and the exhaust passage cover section. A force is applied to the sections during the gluing operation and it has been found that if the force is only applied through the abutting rims, the internal walls of the cylinder block section and exhaust passage cover section will not be adequately bonded, with the result that the internal walls of the metal casting could be defective. On the other hand, if the force is applied over the entire surface of the cover, the thin walled water passage cover section tends to deform inwardly and the abutting external walls may not be properly bonded.
Because of these problems, in obtaining adequate bonding of the abutting walls, it has been the practice to separately bond the exhaust passage cover to the cylinder block section, and after the adhesive is fully set, the water passage cover section is then bonded to the exhaust passage cover section in a separate bonding operation. This procedure of course, substantially increases the overall time required to fabricate the pattern.